Sera from Hawaiian wild and domestic animals were examined by plaque reduction neutralization method for evidence of encephalomyocarditis virus neutralizing antibodies. Of the animals tested, the highest prevalnce of infection was found in Rattus rattus. Infection rates varied widely among rats from different localities. Neutralization tests performed on human sera from various regions of the world indicated a high prevalence of EMC virus antibodies among certain Pacific island populations. Age-specific antibody rates among these groups suggest that infection is acquired in early childhood. Examination of sera from 400 juvenile diabetics and control children failed to demonstrate an association between juvenile diabetes and EMC virus infection. These studies are designed to learn more about the ecology of EMC virus and its disease potential in man.